PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 227 



between states B and A must occur at the crossing point of the potential 

 curves of the two states. However, although the potential curves inter- 

 sect, the energy of the intersection point will match (barring unlikely 

 coincidences) neither a possible vibrational level of state B nor one of 

 state A. Since the transition must be one between states of exactly equal 

 energy, it is forbidden. (If one curve should be repulsive, as in the case 

 of predissociation, the matching is always possible, although perhaps not 

 exactly at the intersection point; the transition is then one between states 

 of equal energy but slightly different interatomic distance, and is possible 

 subject to quantitative modification by the Franck-Condon principle.) 



The process of internal conversion is one of great importance in the 

 photochemistry and radiation chemistry of polyatomic molecules. Its 

 existence was first postulated by Norrish et at. (1934) and its theoretical 

 basis established by Teller (1937). A number of applications have since 

 been recognized (Franck and Livingston, 1941 ; Franck and Sponer, 1948). 



Internal conversion has many features in common with predissociation. 

 They both may have a similar influence on an absorption spectrum and 

 diminish the intensity of fluorescence to the same extent. This is because 

 both processes involve radiationless transitions at intersections of two 

 potential surfaces. It is possible nevertheless, to establish criteria, based 

 on spectroscopic, thermodynamic, and photochemical information, which 

 can distinguish between them in actual cases (Franck and Sponer, 1948). 

 It is important to bear in mind that the chemical consequences of the two 

 processes are usually completely different. Predissociation results in 

 free atoms or radicals, internal conversion in a violently vibrating mol- 

 ecule which can be crudely termed a "hot" molecule. Thus reactions 

 following internal conversion are in a sense a special variety of thermal 

 reaction. 



If several possibiUties are available, internal conversion seems to occur 

 preferentially so as to transform a minimum of electronic into vibrational 

 energy : that electronic state which is the highest one (below the original 

 state) is favored. A common example occurs when Ught absorption 

 brings a molecule to the first excited singlet state, and internal conversion 

 then brings it to the triplet state lying slightly below; the latter state is 

 metastable and can often be detected by observations of phosphorescence 

 (Kasha, 1950). 



Occasionally, internal conversion, following a light absorption process 

 which raises the molecule from the ground state to the lowest, or one of 

 the lowest, excited electronic states, brings the molecule back to the 

 ground electronic state. It is quite possible, however, that the final 

 state (D) is formally an excited state of A which corresponds in actuality 

 to such a different equilibrium configuration of the atom that it has, 

 when vibrationally de-excited, more or less stability. Thus the product 

 of internal conversion may be a tautomer of the original molecule, or 

 even a "stable" isomer (Franck and Livingston, 1941). Some cases of 



